| Days. | |
| 2 | "Brown body" partially formed, the parts of the polypide being still easily recognisable. |
| 5 | Tentacles still just recognisable: appearance of new polypide-bud. |
| 8 | Stage shown in Fig. 235, 2. |
| 11 | Union of apex of stomach with "brown body." |
| 16 | "Brown body" half surrounded by stomach, and preparing to break up (Fig. 235, 3). |
| 21 | "Brown body" broken up into numerous fragments, contained in the alimentary canal of the new polypide. |
| 35 | "Brown body" almost completely absorbed.[[509]] |
These results did not hold good for all the zooecia in a single colony. In some, the "brown body" was not completely got rid of at the end of sixty-eight days, the conclusion of the experiment.
So striking are the facts relating to the "brown bodies" that it has been believed[[510]] that what we have above described as the individual really consists of two kinds of individuals: firstly, the "polypide" or complex of tentacles and digestive organs; and secondly, the "zooecium," or house of the zooid or polypide, corresponding with what has been described above as the body-wall. The one individual, the zooecium, is on this view provided with successive generations of the second kind of individual, the polypide; and these latter function as the digestive organs of the two-fold organism. This view, though fascinating at first sight, is not borne out by an examination of all the facts of the case, especially when the Entoprocta are taken into account.
History.—The history of the Polyzoa, as far as 1856, has been fully treated by Allman in his great work on the Fresh-water Polyzoa;[[511]] but a few words may be said on this subject.
The Polyzoa attracted comparatively little attention before the beginning of the present century. Originally passed over as seaweeds, their real nature was established in connexion with the discovery of the animal nature of corals. So great a revolution could hardly be accepted without a struggle, and even Linnaeus went no further in this direction than to place them in a kind of half-way group of "zoophytes," whose nature was partly animal and partly vegetable. It is hardly necessary to point out that this view has now been abandoned by common consent; and indeed there is no more reason for regarding an animal as showing an approach to the plants because it grows in the external semblance of a seaweed than there would be for supposing a bee-orchid to be allied to the animal kingdom because of the form of its flowers.
But the claims of the Polyzoa to rank as a separate class were by no means admitted with the discovery that they were animals. They were still confounded with Hydroids, Alcyonarians, or Corals until their possession of a complete alimentary canal was recognised as a feature distinguishing them from those animals. This was principally due to the observations of J. V. Thompson[[512]] in Ireland, who introduced the term Polyzoa; and of C. G. Ehrenberg[[513]] in Germany, who proposed the class-name Bryozoa, or moss-like animals.
It is impossible to avoid all mention of the controversy which has raged with regard to these two rival terms. The controversy is for the present at rest, the name Polyzoa being employed by the majority of English writers, amongst whom must be mentioned Allman, Busk, Hincks, and Norman, admittedly authorities of the first rank; while Bryozoa is employed by practically all the Continental writers.
The priority of Thompson's name is unquestioned. While Ehrenberg, however, definitely introduced Bryozoa as the name of a group, Thompson was less precise in this respect, although he states[[514]] that his discovery "must be the cause of extensive alterations and dismemberments in the class with which they [the Polyzoa] have hitherto been associated." Thompson, in fact, clearly understood that the Polyzoa could no longer rank with the Hydroids. The controversy has been summarised by Hincks, in his History of the British Marine Polyzoa,[[515]] where references to other papers on the same subject are given.
The Polyzoa were associated by H. Milne-Edwards with the Tunicata in the group Molluscoidea (Molluscoïdes[[516]]), to which the Brachiopoda were afterwards added by Huxley.[[517]] A knowledge of the development of the Tunicata has, however, shown that these animals must be withdrawn from any association with the other two groups; while there is little real evidence that even the Brachiopods have anything to do with the Polyzoa.
Classification.—The Polyzoa are divided into two sub-classes:—I, the Entoprocta; and II, the Ectoprocta.[[518]] Although the character referred to by these terms is merely the position of the anus with relation to the tentacles,[[519]] there can be no doubt that the two groups differ widely from one another in many important respects. I do not, however, accept the view, maintained by some authors, that the Entoprocta and the Ectoprocta are two separate classes which are not nearly related.